410 research outputs found
Property and strong Property for unital -algebras
In this paper, we will give a thorough study of the notion of Property
for -algebras (as introduced by M.B. Bekka in \cite{Bek-T}) as well as a
slight stronger version of it, called "strong property " (which is also an
analogue of the corresponding concept in the case of discrete groups and type
-factors). More precisely, we will give some interesting equivalent
formulations as well as some permanence properties for both property and
strong property . We will also relate them to certain -type
properties of the unitary group of the underlying -algebra
Linear orthogonality preservers of Hilbert bundles
Due to the corresponding fact concerning Hilbert spaces, it is natural to ask
if the linearity and the orthogonality structure of a Hilbert -module
determine its -algebra-valued inner product. We verify this in the case
when the -algebra is commutative (or equivalently, we consider a Hilbert
bundle over a locally compact Hausdorff space). More precisely, a
-linear map (not assumed to be bounded) between two
Hilbert -modules is said to be "orthogonality preserving" if
\left =0 whenever \left =0. We prove
that if is an orthogonality preserving map from a full Hilbert
-module into another Hilbert -module that
satisfies a weaker notion of -linearity (known as "localness"),
then is bounded and there exists such that
\left\ =\ \phi\cdot\left, \quad \forall
x,y \in E. On the other hand, if is a full Hilbert -module over
another commutative -algebra , we show that a
"bi-orthogonality preserving" bijective map with some "local-type
property" will be bounded and satisfy \left\ =\
\phi\cdot\left\circ\sigma, \quad \forall x,y \in E where and is a homeomorphism
Linear orthogonality preservers of Hilbert -modules over general -algebras
As a partial generalisation of the Uhlhorn theorem to Hilbert -modules,
we show in this article that the module structure and the orthogonality
structure of a Hilbert -module determine its Hilbert -module
structure. In fact, we have a more general result as follows. Let be a
-algebra, and be Hilbert -modules, and be the ideal of
generated by . If is an
-module map, not assumed to be bounded but satisfying then there exists a unique central positive multiplier such
that As a consequence, is automatically bounded, the induced
map is adjointable, and
is isomorphic to as Hilbert -modules. If, in addition,
is bijective, then is isomorphic to .Comment: 15 page
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